1. Field of Invention
This invention relates to processes for the production of alkali metals, especially sodium, and more particularly to electrolytic processes for such production wherein production can be reduced without shutting down the process.
2. Prior Art
Metallic alkali metals are produced generally by the electrolysis of molten halogen salts of such metals, especially mixtures of such salts with other salts which are substantially inert. For example, sodium metal can be produced by electrolysis of a molten binary mixture of calcium chloride and sodium chloride or a ternary mixture of calcium chloride, sodium chloride and barium chloride.
Lithium metal is usually produced by electrolysis of a molten binary eutectic mixture of potassium chloride and lithium chloride.
The type of electrolytic cell most widely used for the above-described operations is the "Downs" cell, which is described in U.S. Pat. No. 1,501,756 to J. C. Downs. Of course, many improvements have been made to this cell over the years; but, basically the Downs-type electrolytic cell is comprised of a refractory lined steel shell for holding the molten salt electrolyte, a submerged cylindrical graphite anode surrounded by a cylindrical steel cathode. A woven wire diaphragm is positioned in the annular space between the electrodes to separate the anode and cathode products, namely halogen gas and the alkali metal. To collect product halogen gas from the anode, the cell is provided with collector means such as an inverted cone which fits over the anode below the surface of the molten electrolyte. Halogen gas (usually chlorine) passes upwardly through the cone and, via appropriate manifold components, from the cell. Similarly, the cathode is also provided with collector means such as an inverted inclined trough which fits over the cathode below the surface of the molten electrolyte. Molten alkali metal rises from the cathode toward the surface of the molten electrolyte, is collected along the inclined surface of the trough and is passed to a vertical riser/cooler pipe in which the molten metal is partially cooled before it is passed to a product receiver.
Commercial Downs cells frequently contain a plurality of electrode assemblies, in which case the anode product collectors are manifolded together to provide a single gas outlet from the cell. Usually, a common cathode product collector is provided which surrounds all of the gas collector cones. That is, the means for collecting the halogen gas is positioned within the perimeter of the molten alkali metal collector. Detailed illustrations and descriptions of such cells can be found in the following U.S. Patents:
______________________________________ R. E. Hulse U.S. 2,130,801 Sept. 20, 1938 J. S. Honea 2,770,364 Nov. 13, 1956 A. L. Fentress 2,770,592 Nov. 13, 1956 G. O. Hoyes 2,944,950 July 12, 1970 C. T. Gallinger 3,037,927 June 5, 1962 G. T. Motock 3,085,967 April 16, 1963 S. E. Eckert and 3,118,827 Jan. 21, 1964 F. J. Ross J. M. Wood 3,248,311 April 26, 1966 L. L. Harris 3,463,721 Aug. 26, 1969 ______________________________________
To reduce alkali metal production from Downs cells, whenever the need arises, for example, a raw material shortage, a strike or over-production, one heretofore could only reduce production either by a limited amount (about 5%) through a tolerable cell amperage reduction or by total cell shutdown. If the current flow is reduced so that production is less than about 95% of its normal value, there is not enough electrical resistance heating of the electrolyte to keep it in the molten state. If the cells are shut down, the electrolyte freezes and the cell must be rebuilt. In the latter case, for instance, a 40% reduction in production means a total shutdown of about 40% of the total operating cells. Total cell shutdown results in a severe financial penalty due to the expense in rebuilding the cells plus the long-term production loss due to the extended period of time required to rebuild the cells. Thus, it has been desirable for quite some time to be able to reduce alkali metal production for a short period of time without affecting product quality, shutting down cells or encountering problems such as polarization, foaming and uncontrollable temperature excursions that result from electrolyte contamination with impurities such as sulfates, boron and soda ash.